Germination of Grass Seed and Pre-emergents

Discussion in 'Turf Renovation' started by LLawn, Nov 13, 2010.

  1. LLawn

    LLawn LawnSite Member
    Messages: 2

    Hey Guys great site. I just stumbled on it the last couple of days and have been pouring over it. Anyways I was wondering how pre-emergent herbicides affect any dormant seeding done in late fall or winter and seed put down in spring. How long must you wait after applying a pre-emergent in spring before laying down grass seed or am I going about it all wrong? Thanks for the help.
  2. fl-landscapes

    fl-landscapes LawnSite Silver Member
    Messages: 2,542

    pre-emergents vary on residual lengths of time depending on which chemical you use and what percentage active ingredient is applied, so read the label of your pre-m and you will have your answer
  3. bx24

    bx24 LawnSite Senior Member
    from MA/TX
    Messages: 503

    I agree here some Pre-M last XX weeks. Some last YY weeks and depends on app. Yea, read the label and find what you need.
  4. RigglePLC

    RigglePLC LawnSite Fanatic
    Messages: 13,800

    It is sometimes a little more complicated. Several factors increase the loss of pre-emergent activity. It depends on what product was used. What rate of active ingredient application per acre. Was it watered-in? The type of soil. The inches of rain. High soil temperature. Degredation by sunlight. Organic matter in soil. And consider the type of seed you plan to use. A large seed will tend to punch through the chemical barrier, (like rye). In most cases, if you use a pre-emergent in spring--you must wait until late August or fall to seed. Most experts agree that seed sown in fall can safely have pre-emergent applied in spring.

    For a late winter or early spring seeding--I am not so sure. In the north if you seed in spring, cold soil (under 50) holds back the germination. But you must have germination and about 2 mowings completed before applying pre-emergent crabgrass control which must go down before soil temp reaches about 60.
    But you are in Virginia...coastal or mountains? What grass are you planning to seed? I am from Michigan, you need local advice for transition and southern grasses.
  5. Hissing Cobra

    Hissing Cobra LawnSite Senior Member
    Messages: 705

    Pre-emergents will definitely affect how the seed comes up. What you need to do is wait until spring to see what came up from the dormant seeding. If it's 2 to 3 inches of blade height, I'd say that you should be o.k. to apply a pre-emergent. If it is not, I wouldn't recommend it. If you're seeding in the spring you won't be able to use a conventional pre-emergent control until the newly sown seed has been mowed twice. By the time this happens, Crabgrass will already be in germination/growing mode and it'll be too late to apply one and have effective results.

    I feel that your best bet is to use a Starter Fertilizer combined with Siduron at the FULL strength label rate (7,500 sq. ft. per 50 lb. bag) at 30 day intervals (2 applications will definitely be needed for MAXIMUM control) until it's full and thick. This product will be EXPENSIVE at approximately $120.00 for a 50 lb. bag but it's the only way for you to grow your grass while keeping Crabgrass growth to the bare minimum. After you've done your two applications (one on the day of seeding and the other 30 days later), you'll be at the 60 day mark. If the lawn has been mowed twice, you can switch over to a conventional Pre-emergent control such as Barricade, Dimension, Pre-M, etc...etc...etc... and this will carry you through the rest of the season. I've mocked up a plan that you could use if you were in Massachusetts. I'm not sure if it would work in your area so you'll have to follow up on that.

    March 15th - Reseed bare, weak areas. Apply 12-24-12 Siduron or equivalent product at Full Strength label rate (7,500 sq. ft per 50lb. bag).

    April 15th - Apply 12-24-12 Siduron or equivalent product at Full Strength label rate (7,500 sq. ft per 50lb. bag).

    May 15th - Analyze lawn. If it's been mowed twice, apply a conventional Pre-emergent control such as Lesco's 19-0-6 Dimension. If it hasn't been mowed twice, reapply the 12-24-12 Siduron product again at this time.

    If you can help it, don't ever seed in the springtime. You'll spend a lot more money on the proper products to ensure that you're successful and it prohibits you from using the cheaper products that are actually better in terms of controlling these pests. The best time to seed every year is September 1st. Seeding at this time allows you to use straight starter fertilizer (NOT combined with Siduron and a lot cheaper). Also, there's approximately 8 weeks left in the season with good growing weather and that's what you'll need to be successful. More importantly, Mother Nature isn't growing half of the crap that she does in the springtime.
    Last edited: Nov 14, 2010
  6. LLawn

    LLawn LawnSite Member
    Messages: 2

    Very helpful guys. I appreciate all the detailed feedback.
  7. Smallaxe

    Smallaxe LawnSite Fanatic
    Messages: 10,082

    The concept of Dormant Seeding involves, settling overwinter, and germinating as soon as conditions are correct. In theory, as soon as soil temps hit 50 degrees F.

    Second point, "Crabgrass Preventer Herbicide" is simply, a root inhibitor. It is fatal only to, immature seedlings and anything with underdeveloped root systems.

    The idea that Crabgrass becomes resistant, to root inhibitor, sooner than cool season grasses, needs closer examination. Dormant seeding allows the seed to germinate in its natural cycle as efficiently as could be expected.

    Proper spring seeding, may also, get cool season grasses going quickly, when soil temps reach 50 degrees F. CG requires 55 - 60 degrees F.

    Here in Wisco, we seldom see CG, in June, at all, in lawns... Lawns shade the soil and maintain cooler temps, than bare soil, as you might see in a garden or sidewalk area. Therefore, one may expect that the springtime growth, of newly germinated cool-season grasses, to be well underway, before the necessity of the application of any root inhibitor, to kill off germinating warm season weeds, such as crab grass.

    The idea of early spring, root inhibitor on lawns, is for commecial profit only. It benefits the applicator, rather than the turf. Spring time is a good time for root growth in mature turf.
    Mature grasses do not require an inhibitor as it breaks dormancy, for good health... Good luck... :)

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